Recent Patents on Electrical Engineering 2008, 1, 178-187
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Advances of LOC-Based Particle Manipulation by AC Electrical Fields Jie Wu* Dept. of Electrical Engineering and Computer Science, The University of Tennessee, Knoxville, TN 37996, USA Received: July 23, 2008; Accepted: August 29, 2008; Revised: September 7, 2008
Abstract: This article presents recent patents and advances on electrical methods for particle manipulation in a microfluidic environment. At microscale, electric fields have unique advantages in performing analytical functions such as manipulation of biological molecules and cells. The advent of AC electrokinetics in recent years further promotes the development of laboratory on a chip, providing versatility and flexibility to interface with many current methods and technologies in multiple biological, chemical and physical disciplines. This article gives an overview of AC electrical principles and their applications, with an emphasis on particle manipulation by electric fields.
Keywords: Microfluidics, electrokinetics, laboratory-on-a-chip, micro total analysis system. 1. INTRODUCTION The concept of laboratory-on-a-chip (LOC), or “micro total analysis system” (GTAS), was proposed in the early 1990s by Manz et al. [1], in which components of a modern laboratory - fluid handling, reactors, heaters, pumps, separators and sensors - are integrated into a single miniature instrument. Since then, the field has grown and branched off with many different applications, such as single cell processing and analysis, biological and chemical analysis, point of care testing, molecular and medical diagnostics, combinatorial chemistry and drug discovery. Traditionally, these biochemical tasks are performed in a laboratory setting by highly qualified personnel, and the equipment is often heavy, bulky and/or too expensive, therefore is mostly not suitable for use in small diagnostic and research laboratories and for decentralized point-of care testing. Hence, there is a great demand for accurate, fast, portable, and low cost analytical/diagnostic tools, which motivates the research and development of LOC. The last twenty years have witnessed significant advances in LOC technologies, and the use of LOCs is widely regarded as one of the key growth industries of the 21st century. Various microfluidic technologies have been developed to perform different functions. Among those, LOC-based techniques to manipulate (bio) particles have broad and versatile impact on modern material technology and life science. Particles manipulation allows the enrichment of molecules of interest from complex samples, and the separation of cells, bacteria, etc. When used as probes for assays, particles offer a huge analytical surface, threedimensional surface, which should improve the micro- (bio) chemical reaction rates. When compared with microarrays, particle-based arrays offer a more flexible choice of the “probe set;” the detection of extra targets only implies the addition of extra microparticles to the sample, while a new microarray has to be made in the case of microarrayassaying. Particle manipulation in LOC-based systems *Address correspondence to this author at the Dept. of Electrical Engineering and Computer Science, The University of Tennessee, Knoxville, TN 37996, USA; Tel: 865-974-5494; Fax: 865-974-5483; E-mail:
[email protected] 1874-4761/08 $100.00+.00
exhibits one salient feature, the size of biological cells or micro particles is in the same order of magnitude as the internal dimensions of a micro-chip system. This enables work with a few or even single cells, and allows the design of very sensitive analytical methods. Many different forces may be employed to manipulate microparticles in a chip, for instance mechanical forces, magnetic forces or optical forces. Electric forces are particularly well suited for miniaturization, because a high field strength may be easily generated with low voltages, and the improvements in micro fabrication where devices incorporate both microelectrodes and microchannels has led to a widespread use of various electrical methods. The aim of this paper is to provide a critical review of the recent electrical technologies available for microparticle manipulation in microchips. 2. ELECTROKINETIC METHODS Many schemes exploiting electrical forces in practical implementations of LOC, known as electrokinetics, are now under investigation. One major category is the particle control schemes, for collecting, separating, positioning, and characterizing suspended biological cells, organelles, or macromolecules. At microscale, electrical fields scale down favorably, low voltages are sufficient to produce intense electrical fields. Thermal and hydrolysis effects that are detrimental for living cells, and that plagued early large-scale electrokinetic devices, can be avoided. There are many other technological advantages in favor of the electrical principles for use in LOC. For example, alternate or direct electric fields are relatively easy to implement and control in microdevices. Elaborate control over the characteristics of the electric fields at length scales comparable to cell size can be achieved by the use of microfabricated electrodes and features. As a consequence of this display of favorable factors, a large number of microscale devices have been proposed for different applications involving the manipulation of cells and particles in suspensions. Electrokinetic manipulations of bioparticles are effective on the length scale of microfluidic systems, and fabrication of microelectrodes is a relatively simple task. Electrokinetic forces have been © 2008 Bentham Science Publishers Ltd.
Recent Patents on AC Electrokinetics
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demonstrated for enhancing mixing, performing sample separation, and improving detection efficiency in microfluidic systems. Many different types of electrokinetic forces can be applied in microfluidic devices. AC electrokinetics (ACEK) has emerged recently for onchip pumping and particle manipulation at low voltage. ACEK has many attractive features. The nonlinear nature of ACEK produces higher transport efficiency than DCEK. ACEK also minimizes undesirable by-products of electrochemical reactions that are unavoidable with DC excitation. Fluid or particle motion generated by ACEK is local, so complex flow patterns can be generated by addressing electrodes individually. This property also makes it possible to manipulate and characterize a single cell or particle. ACEK depends on non-uniform electric fields as well as electric field strength. The most common way to realize nonuniformed electric fields is by adopting asymmetric microelectrodes, and there are numerous variations with three examples shown in Fig. (1) [2]. At small length scale and with microfabricated electrodes, non-uniform electric fields and strong field gradients can be easily achieved without using high voltages. There are mainly three types of ACEK phenomena, dielectrophoresis (DEP) [3-5], AC electroosmosis (ACEO) [6, 7], and AC electrothermal effect (ACET) [8,9]. Dielectrophoresis is the manipulation of particles in nonuniform electric fields. Although ACEO and ACET are techniques for fluid manipulation, they can exert a drag force on particles through fluid motion. Therefore, ACEO and ACET can also be used to manipulate particles, and those methods are generalized as electrofluidic manipulation of particles and discussed later in this article. 3. DIELECTROPHORESIS 3.1. Mechanism The research on Dielectrophoresis (DEP) dates back to 1958 by Pohl’s work [10], and it has revived in the last decade because of the development of microtechnology. As DEP utilizes the interactions between a field-induced dipole moment on a particle and a non-uniform electric field, it has great potential in manipulating and characterizating microparticles. DEP is finding many applications in medical diagnostics, cell therapy etc., and it has evolved into a
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sophisticated and powerful tool for analysis at microscale. A characterization tool for micro-/nano- particles using DEP has been presented by Hughes et al. in patent WO07020443 [11]. Interested readers are referred to Ref. [12] for an overview of cell dielectrophoresis and experimental protocols. DEP induces particle motion by forces arising from the difference in polarizability between the particles and the fluid. A particle with a polarization (conductivity, permittivity) different than that of the suspending medium will influence the electric field distribution around it. As shown by the numerical simulations in Fig. (2), the electric field lines will bend towards the particle if it is more polarized Fig. (2a), and bend away from it if it is less polarized Fig. (2b). If the particle is placed in a non-uniform electric field, the density of electric field lines will be greater on one side of the particle than the other side. This imbalance leads to a net force on the particle, causing it to move, and this effect is known as DEP. A spherical particle experiencing DEP will exhibit a velocity as
u DEP =
a 2 m % p %m a 2 m 2 2 Re Re [ fCM ] E ,
E = 6 6 % p + 2%m (1)
% p, %m are the complex permittivities of the particle p, m ; p,m and p,m and the medium, and % p, m = p, m i where
are the permittivities and conductivities of the particles and the medium, respectively, is the fluid viscosity, and a is the particle diameter. E is the electric field strength. Re means the real part of a complex number. fCM, known as ClausiusMossotti factor, is determined by the difference between the dielectric properties of the particles and surrounding medium [13]. fCM indicates the direction of the DEP force that the particle experiences. When the particle is more polarizable than its surroundings, it will move towards the high field region, known as positive DEP. The opposite situation leads to negative DEP, and the particle moves away from the high field region. Fig. (3) shows two examples of negative and positive DEP [14], respectively, using an electrode design
(b)
(c)
Fig. (1). Some representative electrode structures commonly used in ACEK (a) Side view of cusped electric field; (b) top view of pin-line electrodes; (c) top view of azimuthally periodic electrodes for quadrupolar electric field with zero field magnitude along the central axis [2].
180 Recent Patents on Electrical Engineering, 2008, Vol. 1, No. 3
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Jie Wu
(b)
Fig. (2). Numerically simulated electric field distribution around a sphere in a non-uniform electric field. (a) Sphere more polarizable than the suspending medium, and (b) sphere less polarizable than the suspending medium [13].
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Fig. (3). (a) Negative dielectrophoresis of 557 nm diameter latex spheres on polynomial electrodes for an applied signal of 5 volts peak-topeak at 5 MHz. The spheres can clearly be seen collecting in the low field region. (b) Positive dielectrophoresis of 557 nm diameter latex spheres on polynomial electrodes with an applied signal of 5 volts peak-to-peak at 500 kHz. The spheres collect along the edges of the electrodes at the high field points [14].
similar to that in Fig. (1c). The low field region is at the center of the electrode array and the high field regions are along the electrode edges.
separation, and characterization of particles, such as DNA, protein molecules, virus, bacteria, plant and animal cells, and inorganic particles [3-5].
Because the polarizabilities of particles and media are often frequency dependent, fCM is frequency dependent. Its real part Re(fCM) varies between -0.5 and 1, and consequently DEP velocity will change directions, according to Eq. (1). When Re[fCM] > 0, particles move toward high field regions, e.g. electrode edges, known as positive DEP. When Re[fCM] < 0, particles move away from high field regions, known as negative DEP. Re(fCM) changes its sign at a cross-over frequency, indicated as DEP fxo in Fig. (4). The frequency dependent dielectric properties of particles can be exploited to impose manipulation forces on particles. In a mixture of different particle types, highly effective schemes for particle separation may be realized if one subpopulation of particles expresses a positive DEP effect, while the other exhibits a negative effect. This technique has been studied in great detail for controlled manipulation of particles, binary
The most prevalent biological applications envisioned for DEP are trapping or separation of individual cells or particles, which often rely on the frequency-dependent, dielectric responses of particles. Because microparticles respond differently depending on the frequency of the applied alternate electric fields, it is possible to separate different particle subtypes just by changing the field frequency. In practice, the use of frequency as a control parameter offers an excellent means, so that DEP devices may be easily reconfigured for use with different particles types, which is an advantage over any other type of particle handling devices. 3.2. Particle Enrichment by DEP The essential characteristic of DEP is the movement of objects relative to their suspending medium. Thus, particles
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Fig. (4). Schematic outline of how the DEP crossover frequency DEP fxo is determined from the measurement of cell velocity as a function of the applied electric field frequency. Cell motion towards an electrode edge is defined as positive DEP, whilst negative DEP corresponds to induced motion away from an electrode [15].
can be concentrated or trapped by DEP, and different types of particles can be moved apart from one another under appropriate field conditions. These basic manipulations can be used to sort, isolate, and trap particles, cells and microparticles. Particle trapping using DEP is based on the creation of energy traps of sizes comparable to the particles to be captured. Using microelectrodes, it is relatively easy to generate an electric field at a length scale close to the size of the microparticles, and the field can be localized in a specific area of a microchip, the “particle concentrator.” Such “energy traps” can hold particles against volumetric fluid flow. This technique is generally limited to trapping particles larger than 1 @m, because DEP force is not strong enough to hold smaller particles in place against other forces. DEP force is proportional to the particle volume, and Brownian motion makes it difficult to trap smaller ones, although some reports have described the separation of submicron particles [14]. In order to trap multiple cells in parallel with single-cell resolution, Suehiro and Pethig developed a threedimensional grid electrode system, in which a biological cell can be precisely moved or positioned by positive and negative DEP [15]. The electrode system consisted of two glass plates, on which parallel strip electrodes are fabricated, placed together with a spacer between them. The electrodes face each other and cross at right angles to form the grid. Therefore only 2 n electrodes are needed to control n traps (which significantly simplified fabrication) [16]. Another attractive approach was demonstrated by Chiou et al. who developed a light-induced DEP trap, in which a patterned electrode plate was replaced by a photosensitive film over an electrode. A light image over the photosensitive material will conduct electricity through and creating local electrical fields and hence DEP traps [17]. Each trap could be individually manipulated by programming the projected light images. Chiou et al. demonstrated the trapping of 4.5 @m polystyrene microparticles through the parallel manipulation of 15,000 traps on a 1.3 mm 1.0 mm area.
Non-uniformity of the electric field is the key to DEP trapping. The standard way to create a DEP trap is to generate an electric field gradient with planar metallic electrodes either directly connected to a voltage source or free-floating in an electric field. Banerjee et al. [18] used floating metal posts between electrodes to align carbon nanotubes. On the other hand, insulating materials can also be employed to shape the electric field in a conducting solution to introduce field gradients. An example is so called iDEP by Cummings and Singh [19] who introduced DEP traps by placing an array of insulating posts in a microchannel and applying an electric field across the post, as schematically presented in Fig. (5) [20]. Its principle is the same as that is shown in Fig. (2), electric fields are bended around particles when there is difference in polarizability of the objects and the fluids. Fig. (6) shows the concentration results of live E. coli cells using iDEP device. The circular posts in the array have the following dimensions: 10-m deep, 200-m diameter, and 250 m center-to-center 0°offset, wet etched in glass. Live E. coli cells at a concentration of 6 x 107 cells/mL are labeled green (Syto ® 9 Molecular Probes, Eugene, OR). Flow direction is from right to left. The background electrolyte was deionized water. Based on their success with circular posts, Cummings et al. further suggest to improved the trapping efficiency of their device by incorporating various asymmetric structures in microchannels. The details can be found in US patents [21] and [22]. Another form of DC DEP apparatus is also demonstrated by Li in WO08036082 patent [23]. Typically, traps created by positive DEP are used for separation of particles from flowing mixtures. Particles are trapped are in stable equilibrium and can be released by simply turning off the electric field. Negative DEP forces can be used to spatially confine particles. It can be used for confinement in a static reservoir and in a moving fluid. 3.3. Particle Separation Because the magnitude and direction of DEP forces are related to the particles’ properties, DEP is well suited for
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(a)
Jie Wu
(b)
Fig. (5). Schematic representation of the experimental set-up. (a) Top view, showing the manifold, glass chip, an enlargement of the flow microchannels; (b) cartoon showing the electric field lines being squeezed between the insulating posts [19,20].
Fig. (6). Concentration of live E. coli by using iDEP. The electric fields were: (a) 0 V/mm no concentration of cells; (b) 120 V/mm, concentration of cells; and (c) 160 V/mm, high concentration of cells [19,20].
sorting and separating various types of particles based on their dielectric makeup. For example, a heterogeneous mixture of microparticles in a continuous flow can be spatially separated to produce a more homogeneous population in an appropriate electrical field. The selective capture of particles from a mixture can be enhanced when the frequency of the electric field is chosen such that the unwanted particles are driven away by negative DEP. However, if the crossover frequency for the two types of particles to be separated is close, separation may not be very effective. Depending on the size of the sample, the strategies recommended for dielectrophoretic separation are different. For small samples containing few particles, the particles of interest may be separated by trapping them in predefined locations. For larger samples, where the number of particle
traps would be prohibitively high, particles are usually diverted from the mixture into distinct sub- flow streams based on their dielectric properties and thus separated in time or space. The below briefly describes two DEP separation methods used in a flow through system. FIELD-FLOW FRACTIONATION (FFF) Field-flow fractionation (FFF) applies DEP forces to all particles flowing in microchannels from electrode arrays located at the bottom of the channel. Microparticles mechanically driven through a microdevice by pressure-based flow fields can be separated by a dielectrophoretic force perpendicular to the flow. As shown in Fig. (7), an array of parallel electrodes is deposited on the bottom of a microchannel to generate an inhomogeneous, fringing electric field above the
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Fig. (7). Three major forces acted simultaneously on every particle: sedimentation forces that were proportional to particle weight, DEP forces that fell exponentially with the distance between the cells and the electrodes, and drag forces that were proportional to fluid velocity in the channel. Particles possessing different density or dielectric properties are levitated to different characteristic heights in the flow-velocity profile and attain different velocities V1 and V2 in the chamber, and are thereby fractionated [24].
surface. The magnitude and frequency of the field are chosen such that the two particles are levitated by negative dielectrophoretic forces. Both the electric field strength and field inhomogenity decrease with increasing height above the electrode plane and the DEP force decreases approximately exponentially with height. In addition to the DEP force, a sedimentation force acts on each particle. Dependent on each particle’s individual properties, particles attain different positions relative to the channel bottom where changing levitation balances constant sedimentation forces. Then the particles acquire different velocities corresponding to that height due to the parabolic flow profile. Thus, an initially homogeneous mixture will fractionate; particles carried along by the flow at different rates will emerge at the outlet at different times and captured at different time intervals at the outlet. The method and apparatus for fractionation using dielectrophoresis and field flow fractionation have been demonstrated by Becker et al. in US patent 6641708 [24]. DEFLECTION OF PARTICLES Particle separation may also be achieved by deflecting, rather than focusing, the trajectories of particles in a flow stream according to particle’s dielectric properties [25, 26]. The concept is schematically shown in Fig. (8) with a microchannel and 3-D electrode arrays. Pressure driven flow carries the particles along the channel. AC electric voltage is applied to the electrode pairs facing each other across the microchannel, generating an inhomogeneous electric field which in turn induces dipole moments in the particles. At an appropriate frequency, negative DEP is induced and the particles are repelled from the edge of the electrode pairs Fig. (1b). Whether or not a particle penetrates the dielectrophoretic barrier depends on the relative strength of the dielectrophoretic force and the drag force component exerted on particles at the dielectrophoretic barrier Fig. (1a) while the surrounding fluid is flowing at the speed v. Since dielectrophoretic forces FDEP ~ r3 and hydrodynamic forces FHD ~ r, small particles may penetrate this barrier while large particles experience sufficiently large dielectrophoretic
Fig. (8a). 3-D microelectrode arrays for dielectrophoretic separation of particles in microfluidic channels. Planar electrodes deposited on the top and on the bottom surface of a microchannel form a dielectrophoretic barrier. b) 3-D deflector arrays may be used for particle separation. Size of deflected particles depends on velocity of fluid flow as well as amplitude and frequency of driving voltage [20, 25].
forces to overcome hydrodynamic drag force, FHD, and particles are deflected from the direction of the flow by the action of the dielectrophoretic force and pushed to the side of the channel.
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4. ELECTROFLUIDIC FORCES In a typical DEP concentrator, small electrodes and high voltage are used to generate adequate electric field (in the order of 0.1-1MV/m). However, the force is only effective near the edges of the electrode due to the rapid decay of the electric potential. According to the expression of DEP force, Eq. (1), DEP velocity decreases rapidly with the distance to the electrode, and it is size dependent. Order of magnitude estimation shows that 1 m particles will exhibit no more than 0.9 m/s DEP velocity when they are 10 m from the electrode edge at 5Vrms. On the other hand, fluidic convection has no dependence on particle size and has longer effective range as it takes advantage of the hydrodynamic flow. By employing microflows to convey microparticles, electrofluidic forces concentrate the target samples into a small region in the center of the electrode surface, which is advantageous for a downstream sample preparation or detection process. Electrofluidic forces include AC electroosmosis and AC electrothermal effect, which require only a low applied voltage, meaning only a few volts, to generate bulk fluid motion. 4.1. AC Electroosmosis When a small AC signal is applied over an electrode pair, the electrode surfaces become capacitively charged, i.e. forming counter-ion accumulation, which is referred to as “capacitive charging.” The counter-ions will migrate with or against an electric field that is tangential to the electrode surface, which in turn produces fluid motion due to fluid viscosity. ACEO flow relies on the movement of charges in the electric double layer generated at electrode surfaces when AC signals are applied. Its operating frequency range is typically below 10 kHz so that the induced charges have enough time to follow the transition of electrode polarizability. For conductive fluids, the formation of double layer is greatly compressed and ACEO becomes ineffective manipulating fluids. For the same applied voltage, more conductive fluids have a lower peak ACEO velocity, and the highest fluid conductivity with which ACEO has been observed is 0.085 S/m. 4.2. AC Electrothermal Effect AC Electrothermal (ACET) effect induces fluid motions from the gradients in conductivity and permittivity of the fluid. When an electric field Erms is applied over a fluid body, 2 energy is dissipated as P = Erms (: electrolyte conductivity). A non-uniform electric field will lead to a non-uniform temperature rise, i.e. temperature gradient T , which will produce gradients in conductivity and permittivity as = ( T ) T , = ( T ) T . In turn, and fluid bulk as:
=
generate mobile space charges, , in the
T T T E + i
(2)
The electric field will impose on the induced space charges a force
Jie Wu
Fet = e E
1 2 E 2
(3)
The polarities of the induced charges change with the electric field E, so flow direction can be sustained. Without any external heat sources, ACET velocity exhibits a quartic function (u~V4) with respect to the applied voltage. With an external heat source, such as strong illumination of incident light, the velocity has a quadratic relationship (u~V2), as reported by Castellanos et al. [27]. Recent study has shown that ACET can be generated by electric fields alone in conductive fluids (20~700 mS/m), provided that proper boundary and operating conditions are applied. Sigurdson et al. [28] reported to use ACET effect to induce vortices within a pressure-driven flow-through system (~0.6S/m), improving binding rate of antigenantibody. Lian, et al. [29] also developed two ACET devices without external thermal sources, a parallel plate particle trap and an asymmetric electrode micropump. Both particle trapping and micropumping were demonstrated at low voltages. For fluids with conductivity of 224 mS/m, a peak fluid velocity of ~100 m/s was observed a modest field of 1.6 10 4 Vrms/m at 200 kHz. 4.3. Electrofluidic Particle Concentrator Although ACEO and ACET are techniques for fluid manipulation, they can exert a drag force on particles through fluid motion. Therefore, besides fluid manipulation, ACEO and ACET can also be used to collect and concentrate particles for lab-on-a-chip applications [30-32]. A concentration step is critical when detecting low concentrations of bioparticles, as it can increase particle count at the detection sites to a detectable level. The flow transports the embedded particles in the bulk fluid and pushes the particles to the electrode surface. The bulk fluid flow permits a large effective region for target concentration while other electrokinetic forces allow trapping of the particles into a small region on the electrode surface. The electrokinetic forces, such as DEP and electrophoresis, are especially effective near the electrode surfaces. Therefore, bioparticles can be effectively concentrated into a small region by a combination of fluid flow and particle trapping force. Concentrators of bioparticles, such as cells, virus, proteins, and DNA molecules, using electrofluidic forces with DEP have been prototyped in various forms [33-35]. For example, an ACEO concentrator with side-by-side electrode design is shown in Fig. (9) [33]. Electrofluidic force generates microflows that convey particles from the bulk of the fluid onto the electrode surface. At locations of low flow velocity, particles become adhered to substrates from a variety of forces such as gravity, electrostatic attraction and DEP. By taking advantage of microfabrication technology, various particle assemblies can be formed using different electrode designs, such as the parallel plate electrode design as shown in Fig. (10). The two electrodes are face-to-face and of different size (or with patterns), so tangential electric fields are generated at the edge of the smaller electrode. Microflows are generated towards the center of the electrode,
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Fig. (9a). An AC electroosmotic particle concentrator (top view). (b) Schematic (side view) illustrating electrode polarization and formation of ac electroosmotic flow. Solid arrows represent the AC electroosmotic force and dotted lines indicate the flow pattern [33].
Fig. (10). Trapping microparticle with a pair of electrodes in parallel to each other [36].
conveying particles from the bulk of the fluid onto the electrode surface. Since tangential fields reduce to zero at the center of the electrode, the flows slow down to stagnation and deposit particles. Concentrators with this design have been demonstrated with both ACEO [34] and ACET effect [35]. With either ACEO or ACET mechanism, in order to effectively convect particles to the electrode surface and deposit them there, it is important there are converging flows towards the stagnation location and the flows are not too strong that the upward force outbalances the gravity, DEP and other surface forces. Because particle focusing is realized through fluid motion, it does not depend on particle size or charge, given that particles stay suspended in the fluid long enough (~smaller than 5 m particles) to be focused. The parallel plate design is easy to be incorporated into other devices. Fig. (11) shows an example of its integration with a microcantilever sensor [36]. The ITO (Indium Tin Oxide) glass slide was used as the top electrode and metal coated cantilevers are the other electrode Fig. (11a). Since particles will be collected at conductive surface, places where particle deposition is not desired are covered by dielectrics. The locations where particles are attracted to can be manipulated by patterning of the metal layer or its dielectric coating on the cantilever, so particles can be focused onto the tips or any other locations of cantilevers for maximum sensitivity. Such a device has been successfully implemented with concentration effect shown in Fig. (11b). 4.4. Particle Line Assembly by ACEO AC surface flow has been observed to play an important role in colloidal self-assembly on electrodes [37, 38]. For such applications, ACEO devices typically adopt co-planar thin film electrodes, of which the electric field distribution is
inherently non-uniform with both normal and tangential components. The tangentials of local electric fields change directions over the span of one electrode. As a result, the surface EO flows switch directions over one electrode, and four counter-rotating vortices are produced as drawn schematically in Fig. (12a). The change in the tangential field direction happens at approximately 1 2 of electrodewidth away from the electrode inner edge [7], where the stagnation of microflows takes place. As particles carried by microflows approach the stagnation position, they lose momentum due to counter flows and settle at the electrode surface. This phenomenon can also be explored to attract bioparticles from the bulk of a suspension to electrode surface, thus concentrating bioparticles to the detectors in a short time. The four vortices predicted in Fig. (12a) have been experimentally observed, and Fig. (12b) shows a frame of polystyrene particles in forming lines by 2 Vpp at 500 Hz. A further improvement of ACEO line assembly is to add a DC offset to the AC signal, which is known as “biased ACEO.” It was found that electrochemical reactions at the electrodes also can generate ACEO [39, 40]. The electrode process is referred to as “Faradaic charging,” as opposed to “capacitive charging” discussed earlier. In biased ACEO, electrode pairs are energized by AC signals with sufficient DC offset, so that peak positive potential exceeds the threshold for electrochemical reactions. Consequently, the electrodes in a pair undergo differential surface charging, i.e. the positively-biased electrode electrochemically produces co-ions (left electrode in Fig. 13a) and the negatively-biased electrode capacitively attracts counter-ions (right electrode in Fig. 13a). With biased ACEO, the charges produced at the both electrodes are of the same polarity, and tangential electric fields are in the same direction for the most part of electrodes. Consequently, an unidirectional flow is induced
186 Recent Patents on Electrical Engineering, 2008, Vol. 1, No. 3
Jie Wu
(a)
(b)
Fig. (11a). schematic of a microcantilever particle concentrator (b) 1 m fluorescent particles being focused onto the central parts of cantilevers [36].
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(b)
Fig. (12a). Four counter-rotating vortices are formed above the electrodes due to changes in tangential electric fields. At an appropriate strength of electrode polarization, particles aggregate at the stagnation points. (b) Two lines of concentrated latex particles are formed on the electrodes by capacitive charging at low potentials [37, 38].
(a)
(b)
Fig. (13a). At an appropriately biased AC potential, asymmetric vortices are formed above two electrodes, streamlines from capacitive charging and Faradaic charging become connected, forming a large vortex over the electrode pair. Particles can be moved from the right to the left electrode. (13b) Particles move from the right to the left electrodes at higher potentials [41, 42].
at the electrode surface, generating a large vortex to convect particles, as shown in Fig. (13a & b) [41,42]. With biased ACEO, particles can be concentrated onto a designated electrode. What is also important for biased ACEO is that electro-phoretic/electrostatic force is exerted simultaneously with ACEO to move bioparticles towards positively-biased electrodes. Because most bioparticles are negatively charged, biased ACEO exhibits much more robust and effective trapping of particles than regular, unbiased ACEO. Experiments have demonstrated superior capability of biased ACEO in concentrating particles [7, 36]. The benefits are
two-folds, higher local particle density and stronger adhesion of the particles due to additional electrostatic force. The technique of biased ACEK has been included in US patent application 20080032326 to enhance the detection of toxic agents in waters [43]. 5. CURRENT & FUTURE DEVELOPMENTS The last decade has witnessed a steady expansion in the new methods and devices associated with LOC technologies, and microfluidic devices are nowadays considered as a common aid to various applications in natural and life
Recent Patents on AC Electrokinetics
sciences. The versatility of ACEK devices allows interfacing with many current methods and technologies. It is evident that the inherent flexibility of ACEK technologies will allow them to permeate and advance the development of lab-on-achip. The purpose of this review is to assist newcomers to the field by giving a broad overview of the novel achievements. As a result, for every distinct method or device, only some relevant examples were given in the review. 6. ACKNOWLEDGMENT JW acknowledges the support of National Science Foundation, USA, under the grant No. 0448896.
Recent Patents on Electrical Engineering, 2008, Vol. 1, No. 3 [18]
[19] [20]
[21] [22] [23] [24] [25]
REFERENCES [1] [2] [3]
[4] [5] [6]
[7]
[8]
[9] [10] [11] [12] [13] [14]
[15]
[16] [17]
Manz A, Grabber N, Widmer HM. Miniaturized total chemical analysis systems: A novel concept for chemical sensing. Sens Actuat BI 1990; 244-248. Jones TB. Basic theory of dielectrophoresis and electrorotation. IEEE Eng Med Biol Mag 2003; 22(6): 33-42. Pethig R. Application of A.C. electrical fields to the manipulation and characterization of cells. In: Karube I, Ed., Automation in Biotechnology, Elsevier, 1991; 159-185. Pohl HA. Dielectrophoresis, Cambridge University Press, Cambridge, 1995. Gascoyne PRC, Vykoukal JV. Dielectrophoresis-based sample handling in general-purpose programmable diagnostic instruments. Proc IEEE 2004; 92: 22-42. Green NG, Ramos A, Gonzalez A, Morgan H, Castellanos A. Fluid flow induced by nonuniform ac electric fields in electrolytes on microelectrodes. I. Experimental measurements. Phys Rev E 2000; 61: 4011-4018. Green NG, Ramos A, Gonzalez A, Morgan H, Castellanos A. Fluid flow induced by nonuniform ac electric fields in electrolytes on microelectrodes. III. Observation of streamlines and numerical simulation. Phys Rev E 2002; 66: 026305. Ramos A, Morgan H, Green NG, Castellanos A. AC electrokinetics: A review of forces in microelectrode structures. J Phys D Appl Phys 1998; 31: 2338-2353. Green NG, Ramos A, Gonzalez A, Castellanos A, Morgan H. Electrothermally induced fluid flow on microelectrodes. J Electrostat 2001; 53: 71-87. Pohl HA. Some effects of nonuniform fields on dielectrics. J Appl Phys 1958; 29(8): 1182-1188. Hughes, M., Hoettges, K.F., Morgan, H.A.: WO07020443 (2007). Pethig R. Dielectrophoresis of biological cells. In: Somasundaran P, Ed., Encyclopedia of Surface and Colloid Science, CRC Press 2006, 1719-1736. Wu J, Interactions of electrical fields with fluids: Laboratory-on-achip Applications. IET Nanobiotechnol 2008, 2(1): 14-27. Green NG, Ramos A, Morgan H. AC electrokinetics: a survey of sub-micrometre particle dynamics. J Phys D Appl Phys 2000; 33: 632-641. Pethig R, Bressler V, Carswell-Crumpton C, et al. Dielectrophoretic studies of the activation of human T lymphocytes using a newly developed cell profiling system. Electrophoresis 2002; 23: 2057-2063. Junya S, Pethig R. The dielectrophoretic movement and positioning of a biological cell using a three-dimensional grid electrode system. J Phys D Appl Phys 1998; 31: 3298-3305. Chiou PY, Aaron TO, Wu MC. Massively parallel manipulation of single cells and microparticles using optical images. Nature 2005; 436: 370-372.
[26] [27] [28] [29] [30]
[31] [32] [33]
[34] [35]
[36] [37] [38]
[39] [40]
[41] [42] [43]
187
Banerjee S, White BE, Huang L, Rego BJ, O’Brien S, Hermana IP. Precise positioning of single-walled carbon nanotubes by ac dielectrophoresis, J Vac Sci Technol 2006; 24 B (6): 3173-3178. Cummings EB, Singh A. Dielectrophoresis in microchips containing arrays of insulating posts: Theoretical and experimental results. Anal Chem 2003; 75(18): 4724-4731. Simmons BA, Cummings EB, Davalos RV, et al. Separation and Concentration of Water-Borne Contaminants Utilizing InsulatorBased Dielectrophoresis, technical report, www.osti.gov/ energycitations/servlets/purl/878575-avY8KU/878575.PDF. Cummings, E.B., Fintschenko, Y., Simmons, B.: US20047419574 (2004). Cummings, E.B, Fiechtner, G.J.: US20037347923 (2003). Li, D.: WO08036082 (2008). Becker, F.F., Gascoyne, P.R.C., Huang, Y., Wang, X., Yang, J.: US20036641708 (2003). Dürr M, Kentsch J, Müller T, Schnelle T, Stelzle M. Microdevices for manipulation and accumulation of micro- and nanoparticles by dielectrophoresis, Electrophoresis 2003; 24: 722-731. Chang H, Ikram A, Kosari F, Vasmatzis G, Bhunia A, Bashir R. Electrical characterization of micro-organisms using microfabricated devices. J Vac Sci Technol B 2002; 20: 2058-2064. Olesen LH, Bruus H, Ajdari A. Phys Rev E 2006; 73: 056313. Sigurdson M, Wang DZ, Meinhart CD. Electrothermal stirring for heterogeneous immunoassays. Lab Chip 2005; 5: 1366-1373. Lian M, Islam N, Wu J. AC electrothermal manipulation of conductive fluids and particles for lab-chip applications. IET Nanobiotechnol 2007; 1(3): 36-43. Hoettges KF, McDonnel MB, Hughes MP. Use of combined dielectrophoretic/ electrohydrodynamic forces for biosensor enhancement. J Phys D Appl Phys 2003; 36: L101-104. Wong, PK, Chen C-Y, Wang T-H, Ho C-M. An AC electroosmotic processor for biomolecules. Transducers 2003; 20-23. Wu J, Islam N, Lian M. High sensitivity particle detection by biased ac electro-osmotic trapping on cantilever. 19th IEEE Int’l Conf. Micro Electro Mechanical Systems (MEMS) 2006; 566-569. Wong PK, Chen CY, Wang TH, Ho CM. Electrokinetic bioprocessor for concentrating cells and molecules. Anal Chem 2004; 76 (23): 6908-6914. Bhatt KH, Sonia G, Velev OD. An AC electrokinetic technique for collection and concentration of particles and cells on patterned electrodes. Langmuir 2005; 21 (14): 6603 -6612. Lian M, Islam N, Wu J. AC electrothermal manipulation of conductive fluids and particles for lab-chip applications. IET Nanobiotechnol 2007; 1(3): 36-43. Wu J, Islam N. A simple method to integrate in-situ nano-particle focusing with cantilever detection.” IEEE Sens J 2007; 7(6): 957958. Trau M, Saville DA, Aksay IA. Field-induced layering of colloidal crystals. Science 1996; 272: 706-709. Yeh SR, Seul M, Shraiman BI. Assembly of ordered colloidal aggregrates by electric-field-induced fluid flow. Nature 1997; 386: 57-59. Wu J, Ben Y, Chang HC. Particle detection by micro- electrical impedance spectroscopy with asymmetric-polarization AC electroosmotic trapping. J Microfluid Nanofluid 2005; 1(2): 161-167. Wu J, Ben Y, Battigelli D and Chang HC. Long-Range AC electrokinetic trapping and detection of bioparticles. Industr Eng Chem Res 2005; 44(8): 2815-2822. Wu J. Electrokinetic microfluidics for on-chip bioparticle processing. IEEE Trans Nanotech 2006; 5(2): 84-89. Wu J. AC Electroosmotic micropump by asymmetric electrode polarization. J Appl Phys 2008; 103: 024907. Greenbaum, E., Rodriguez, M., Wu, J., Qi, H.: US20080032326 (2008).
